It sounds like you have hit the dreaded plateau - it happens to practically everyone sooner or later. Stick to you guns! Have you been measureing yourself? Lots of people find that even though the scale fairy is not being very fair-like, you may be losing inches anyway as your body adjusts and rearranges to you new weight.
The other thing that will certainly help at this point is to find an exercise routine that you like. Walking is by far one of the best at this point in your journey. If you can find a friend to walk with, so much the better. You might also try looking into videos. Your local library may have a good selection. Start slow so you don't burn out early, and don't be afraid to check out lots of different types until you find one that clicks.
There are lots of ladies here who are exercise video experts, so hopefully some will weigh in with some suggestions.
Since you are so successfull, you may also want to (if you haven't already) start adopting some of those hidden exercise tricks like parking at the back of lot when you go shopping, always taking the stairs, or packing the grocery bags just a little bit heavier than usual. Someone had a great suggestions of always sitting on the floor when they watch TV. It is much harder to get up from the floor than from the couch
In any event keep it up girl! You will get there. The ups and downs are just part of the journey.